Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from having drop ejectors which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially the ink bulges from the channel orifice. The bubble quickly collapses and the ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper can be stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of a sheet of recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959.
Printers print information received from an image output device such as a personal computer. Oftentimes, this received information is in the form of a raster scan image such as a full page bitmap or in the form of an image written in a page description language. The raster scan image includes a series of the scan lines or rows consisting of bits representing pixel information in which each scan line or row contains information sufficient to print a single fine line of information across a page in a linear fashion. Printers can print bitmap information as received. If a printer receives an image written in the page description language, however, the printer or the image input device converts the page description language to a bitmap consisting of pixel information.
The density of information contained in the full page bitmap can correspond to the density of the image to be printed by the liquid ink printer. For instance, in a thermal ink jet printhead printing at 300 spots per inch, the full page bitmap will have information enabling the printhead to print at the required density. Known printers also manipulate image bitmaps to print at resolutions greater than or less than the resolution of the received image.
In reciprocating carriage printers, image defects can occur due to non-uniform absorption and drying of the ink. These image defects can be reduced by printing the image in more than one pass of the printhead, wherein each pass prints a portion of the pixels in a dot pattern known as a "checkerboard" pattern. In this type of two pass printing, a first pass of the printhead carriage prints a swath of information in which odd numbered pixels of odd numbered rows or scanlines and even numbered pixels of even numbered rows or scanlines of a bitmap are printed. In a second pass of the carriage printhead, the complementary pattern consisting of even numbered pixels in odd numbered rows and odd numbered pixels in even numbered rows is printed. By printing in two passes, the ink printed in the first pass has time to dry partially before the ink from the second pattern is deposited.
Printing in passes of the printhead can, however, produce a print defect known as "banding". Banding is the phenomenon which results from printing one swath of information next to another swath of information such that no overlap of adjacent swaths occurs and a line or band is apparent between adjacent swaths. Banding may also occur if the printhead ejects drops of varying sizes and in different firing directions due to directionality problems. To reduce or prevent banding, checkerboard patterns are typically printed in overlapping swaths. While conventional checkerboarding can reduce image defects and overlapping of checkerboard swaths can reduce or eliminate banding image defects, further improvements are desirable since complete or partial overlapping of checkerboard patterns may not reduce image defects to a desirable level. The following references describe these and other problems associated with liquid ink printing and provide a variety of solutions for these known problems.
U.S. Pat. No. 4,748,453 to Lin et al., describes a method of depositing spots of liquid ink upon selected pixel centers on a substrate having poor ink absorptive properties. The lines of information are printed in at least two passes so that liquid ink is deposited on selected pixel centers in a checkerboard pattern. In a second pass, the complementary checkerboard pattern is deposited over the first checkerboard pattern.
U.S. Pat. No. 4,999,646 to Trask describes a multiple pass complementary dot pattern ink jet printing process. Successive printed swaths of information are made by depositing a first dot pattern and a second partially overlapping complementary dot pattern on the print media. The adjacent spacing of dots is alternated in coincident dot rows and the first and second dot patterns are overlapped by a predetermined percentage which is less than 100%.
Japanese Laid-Open No. 60-107975, entitled "Ink Jet Recording Apparatus" describes an ink jet recording apparatus having a recording head scanning in a lateral direction wherein a first main scan and a second main scan overlap. The array of print dots in the overlapping area are printed only once. The dots within the array of dots are printed by either the first main scan or the second main scan wherein odd rows of the overlapping area are printed by the first scan and even rows are printed by the second scan, every other column of the odds rows and the other columns of the even rows are printed by the first scan and remaining dots are printed by the second scan, or the overlapping area is elected at random to be printed by the first main scan or the second main scan.
U.S. Pat. No. 4,967,203 to Doan et al. describes an interlace printing process for an ink jet printer. Printed images are produced by staggering applications of ink dots to pixel locations such that overlapping ink dots are printed on successive passes of a printhead and such that swaths are partially printed on overlapping passes of the printhead. Multi-colored or multi-shaded images are completed by grouping pixels into superpixels and applying various combinations of colored ink dots to the various pixels within each superpixel in a staggered sequence.
U.S. Pat. No. 5,012,257 to Lowe et al. describes ink jet color graphics printing where an image superpixel consists of a 2.times.2 array of cells wherein each cell corresponds to a pixel area on a substrate. Each pixel of graphics data is processed to form a 2.times.2 array of bit image data for printing a corresponding superpixel image. A superpixel configuration indicating cell location and color of drops of ink for forming a superpixel image is defined for each desired image color.